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<H1 align=center>Apache Tutorial: Introduction to Server Side Includes</H1><A 
id=__index__ name=__index__></A><!-- INDEX BEGIN -->
<UL>
  <LI><A 
  href="http://httpd.apache.org/docs/howto/ssi.html#apachetutorial:introductiontoserversideincludes">Apache 
  Tutorial: Introduction to Server Side Includes</A> 
  <LI><A href="http://httpd.apache.org/docs/howto/ssi.html#whataressi">What are 
  SSI?</A> 
  <LI><A 
  href="http://httpd.apache.org/docs/howto/ssi.html#configuringyourservertopermitssi">Configuring 
  your server to permit SSI</A> 
  <LI><A 
  href="http://httpd.apache.org/docs/howto/ssi.html#basicssidirectives">Basic 
  SSI directives</A> 
  <UL>
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#today'sdate">Today's 
    date</A> 
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#modificationdateofthefile">Modification 
    date of the file</A> 
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#includingtheresultsofacgiprogram">Including 
    the results of a CGI program</A> </LI></UL>
  <LI><A 
  href="http://httpd.apache.org/docs/howto/ssi.html#additionalexamples">Additional 
  examples</A> 
  <UL>
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#whenwasthisdocumentmodified">When 
    was this document modified?</A> 
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#includingastandardfooter">Including 
    a standard footer</A> 
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#whatelsecaniconfig">What 
    else can I config?</A> 
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#executingcommands">Executing 
    commands</A> </LI></UL>
  <LI><A 
  href="http://httpd.apache.org/docs/howto/ssi.html#advancedssitechniques">Advanced 
  SSI techniques</A> 
  <UL>
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#settingvariables">Setting 
    variables</A> 
    <LI><A 
    href="http://httpd.apache.org/docs/howto/ssi.html#conditionalexpressions">Conditional 
    expressions</A> </LI></UL>
  <LI><A 
  href="http://httpd.apache.org/docs/howto/ssi.html#conclusion">Conclusion</A> 
  </LI></UL><!-- INDEX END -->
<HR>

<H2><A id=apachetutorial:introductiontoserversideincludes 
name=apachetutorial:introductiontoserversideincludes>Apache Tutorial: 
Introduction to Server Side Includes</A></H2>
<TABLE border=1>
  <TBODY>
  <TR>
    <TD vAlign=top><STRONG>Related Modules</STRONG><BR><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_include.html">mod_include</A><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_cgi.html">mod_cgi</A><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_expires.html">mod_expires</A><BR></TD>
    <TD vAlign=top><STRONG>Related Directives</STRONG><BR><BR><A 
      href="http://httpd.apache.org/docs/mod/core.html#options">Options</A><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_include.html#xbithack">XBitHack</A><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_mime.html#addtype">AddType</A><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_mime.html#addhandler">AddHandler</A><BR><A 
      href="http://httpd.apache.org/docs/mod/mod_setenvif.html#BrowserMatchNoCase">BrowserMatchNoCase</A><BR></TD></TR></TBODY></TABLE>
<P>This HOWTO first appeared in Apache Today (http://www.apachetoday.com/) as a 
series of three articles. They appear here by arrangement with ApacheToday and 
Internet.com.</P>
<P>This article deals with Server Side Includes, usually called simply SSI. In 
this article, I'll talk about configuring your server to permit SSI, and 
introduce some basic SSI techniques for adding dynamic content to your existing 
HTML pages.</P>
<P>In the latter part of the article, we'll talk about some of the somewhat more 
advanced things that can be done with SSI, such as conditional statements in 
your SSI directives.</P>
<HR>

<H2><A id=whataressi name=whataressi>What are SSI?</A></H2>
<P>SSI (Server Side Includes) are directives that are placed in HTML pages, and 
evaluated on the server while the pages are being served. They let you add 
dynamically generated content to an existing HTML page, without having to serve 
the entire page via a CGI program, or other dynamic technology.</P>
<P>The decision of when to use SSI, and when to have your page entirely 
generated by some program, is usually a matter of how much of the page is 
static, and how much needs to be recalculated every time the page is served. SSI 
is a great way to add small pieces of information, such as the current time. But 
if a majority of your page is being generated at the time that it is served, you 
need to look for some other solution.</P>
<HR>

<H2><A id=configuringyourservertopermitssi 
name=configuringyourservertopermitssi>Configuring your server to permit 
SSI</A></H2>
<P>To permit SSI on your server, you must have the following directive either in 
your <CODE>httpd.conf</CODE> file, or in a <CODE>.htaccess</CODE> file:</P><PRE>        Options +Includes
</PRE>
<P>This tells Apache that you want to permit files to be parsed for SSI 
directives. Note that most configurations contain multiple <A 
href="http://httpd.apache.org/docs/mod/core.html#options">Options</A> directives 
that can override each other. You will probably need to apply the 
<CODE>Options</CODE> to the specific directory where you want SSI enabled in 
order to assure that it gets evaluated last.</P>
<P>Not just any file is parsed for SSI directives. You have to tell Apache which 
files should be parsed. There are two ways to do this. You can tell Apache to 
parse any file with a particular file extension, such as <CODE>.shtml</CODE>, 
with the following directives:</P><PRE>        AddType text/html .shtml
        AddHandler server-parsed .shtml
</PRE>
<P>One disadvantage to this approach is that if you wanted to add SSI directives 
to an existing page, you would have to change the name of that page, and all 
links to that page, in order to give it a <CODE>.shtml</CODE> extension, so that 
those directives would be executed.</P>
<P>The other method is to use the <CODE>XBitHack</CODE> directive:</P><PRE>        XBitHack on
</PRE>
<P><CODE>XBitHack</CODE> tells Apache to parse files for SSI directives if they 
have the execute bit set. So, to add SSI directives to an existing page, rather 
than having to change the file name, you would just need to make the file 
executable using <CODE>chmod</CODE>.</P><PRE>        chmod +x pagename.html
</PRE>
<P>A brief comment about what not to do. You'll occasionally see people 
recommending that you just tell Apache to parse all <CODE>.html</CODE> files for 
SSI, so that you don't have to mess with <CODE>.shtml</CODE> file names. These 
folks have perhaps not heard about <CODE>XBitHack</CODE>. The thing to keep in 
mind is that, by doing this, you're requiring that Apache read through every 
single file that it sends out to clients, even if they don't contain any SSI 
directives. This can slow things down quite a bit, and is not a good idea.</P>
<P>Of course, on Windows, there is no such thing as an execute bit to set, so 
that limits your options a little.</P>
<P>In its default configuration, Apache does not send the last modified date or 
content length HTTP headers on SSI pages, because these values are difficult to 
calculate for dynamic content. This can prevent your document from being cached, 
and result in slower perceived client performance. There are two ways to solve 
this:</P>
<OL>
  <LI>Use the <CODE>XBitHack Full</CODE> configuration. This tells Apache to 
  determine the last modified date by looking only at the date of the originally 
  requested file, ignoring the modification date of any included files. 
  <LI>Use the directives provided by <A 
  href="http://httpd.apache.org/docs/mod/mod_expires.html">mod_expires</A> to 
  set an explicit expiration time on your files, thereby letting browsers and 
  proxies know that it is acceptable to cache them. </LI></OL>
<HR>

<H2><A id=basicssidirectives name=basicssidirectives>Basic SSI 
directives</A></H2>
<P>SSI directives have the following syntax:</P><PRE>        &lt;!--#element attribute=value attribute=value ... --&gt;
</PRE>
<P>It is formatted like an HTML comment, so if you don't have SSI correctly 
enabled, the browser will ignore it, but it will still be visible in the HTML 
source. If you have SSI correctly configured, the directive will be replaced 
with its results.</P>
<P>The element can be one of a number of things, and we'll talk some more about 
most of these in the next installment of this series. For now, here are some 
examples of what you can do with SSI</P>
<H3><A id="today'sdate" name="today'sdate">Today's date</A></H3><PRE>        &lt;!--#echo var="DATE_LOCAL" --&gt;
</PRE>
<P>The <CODE>echo</CODE> element just spits out the value of a variable. There 
are a number of standard variables, which include the whole set of environment 
variables that are available to CGI programs. Also, you can define your own 
variables with the <CODE>set</CODE> element.</P>
<P>If you don't like the format in which the date gets printed, you can use the 
<CODE>config</CODE> element, with a <CODE>timefmt</CODE> attribute, to modify 
that formatting.</P><PRE>        &lt;!--#config timefmt="%A %B %d, %Y" --&gt;
        Today is &lt;!--#echo var="DATE_LOCAL" --&gt;
</PRE>
<H3><A id=modificationdateofthefile name=modificationdateofthefile>Modification 
date of the file</A></H3><PRE>        This document last modified &lt;!--#flastmod file="index.html" --&gt;
</PRE>
<P>This element is also subject to <CODE>timefmt</CODE> format 
configurations.</P>
<H3><A id=includingtheresultsofacgiprogram 
name=includingtheresultsofacgiprogram>Including the results of a CGI 
program</A></H3>
<P>This is one of the more common uses of SSI - to output the results of a CGI 
program, such as everybody's favorite, a ``hit counter.''</P><PRE>        &lt;!--#include virtual="/cgi-bin/counter.pl" --&gt;
</PRE>
<HR>

<H2><A id=additionalexamples name=additionalexamples>Additional 
examples</A></H2>
<P>Following are some specific examples of things you can do in your HTML 
documents with SSI.</P>
<HR>

<H2><A id=whenwasthisdocumentmodified name=whenwasthisdocumentmodified>When was 
this document modified?</A></H2>
<P>Earlier, we mentioned that you could use SSI to inform the user when the 
document was most recently modified. However, the actual method for doing that 
was left somewhat in question. The following code, placed in your HTML document, 
will put such a time stamp on your page. Of course, you will have to have SSI 
correctly enabled, as discussed above.</P><PRE>        &lt;!--#config timefmt="%A %B %d, %Y" --&gt;
        This file last modified &lt;!--#flastmod file="ssi.shtml" --&gt;
</PRE>
<P>Of course, you will need to replace the <CODE>ssi.shtml</CODE> with the 
actual name of the file that you're referring to. This can be inconvenient if 
you're just looking for a generic piece of code that you can paste into any 
file, so you probably want to use the <CODE>LAST_MODIFIED</CODE> variable 
instead:</P><PRE>        &lt;!--#config timefmt="%D" --&gt;
        This file last modified &lt;!--#echo var="LAST_MODIFIED" --&gt;
</PRE>
<P>For more details on the <CODE>timefmt</CODE> format, go to your favorite 
search site and look for <CODE>ctime</CODE>. The syntax is the same.</P>
<HR>

<H2><A id=includingastandardfooter name=includingastandardfooter>Including a 
standard footer</A></H2>
<P>If you are managing any site that is more than a few pages, you may find that 
making changes to all those pages can be a real pain, particularly if you are 
trying to maintain some kind of standard look across all those pages.</P>
<P>Using an include file for a header and/or a footer can reduce the burden of 
these updates. You just have to make one footer file, and then include it into 
each page with the <CODE>include</CODE> SSI command. The <CODE>include</CODE> 
element can determine what file to include with either the <CODE>file</CODE> 
attribute, or the <CODE>virtual</CODE> attribute. The <CODE>file</CODE> 
attribute is a file path, <EM>relative to the current directory</EM>. That means 
that it cannot be an absolute file path (starting with /), nor can it contain 
../ as part of that path. The <CODE>virtual</CODE> attribute is probably more 
useful, and should specify a URL relative to the document being served. It can 
start with a /, but must be on the same server as the file being served.</P><PRE>        &lt;!--#include virtual="/footer.html" --&gt;
</PRE>
<P>I'll frequently combine the last two things, putting a 
<CODE>LAST_MODIFIED</CODE> directive inside a footer file to be included. SSI 
directives can be contained in the included file, and includes can be nested - 
that is, the included file can include another file, and so on.</P>
<HR>

<H2><A id=whatelsecaniconfig name=whatelsecaniconfig>What else can I 
config?</A></H2>
<P>In addition to being able to <CODE>config</CODE> the time format, you can 
also <CODE>config</CODE> two other things.</P>
<P>Usually, when something goes wrong with your SSI directive, you get the 
message</P><PRE>        [an error occurred while processing this directive]
</PRE>
<P>If you want to change that message to something else, you can do so with the 
<CODE>errmsg</CODE> attribute to the <CODE>config</CODE> element:</P><PRE>        &lt;!--#config errmsg="[It appears that you don't know how to use SSI]" --&gt;
</PRE>
<P>Hopefully, end users will never see this message, because you will have 
resolved all the problems with your SSI directives before your site goes live. 
(Right?)</P>
<P>And you can <CODE>config</CODE> the format in which file sizes are returned 
with the <CODE>sizefmt</CODE> attribute. You can specify <CODE>bytes</CODE> for 
a full count in bytes, or <CODE>abbrev</CODE> for an abbreviated number in Kb or 
Mb, as appropriate.</P>
<HR>

<H2><A id=executingcommands name=executingcommands>Executing commands</A></H2>
<P>I expect that I'll have an article some time in the coming months about using 
SSI with small CGI programs. For now, here's something else that you can do with 
the <CODE>exec</CODE> element. You can actually have SSI execute a command using 
the shell (<CODE>/bin/sh</CODE>, to be precise - or the DOS shell, if you're on 
Win32). The following, for example, will give you a directory listing.</P><PRE>        &lt;pre&gt;
        &lt;!--#exec cmd="ls" --&gt;
        &lt;/pre&gt;
</PRE>
<P>or, on Windows</P><PRE>        &lt;pre&gt;
        &lt;!--#exec cmd="dir" --&gt;
        &lt;/pre&gt;
</PRE>
<P>You might notice some strange formatting with this directive on Windows, 
because the output from <CODE>dir</CODE> contains the string 
``&lt;<CODE>dir</CODE>&gt;'' in it, which confuses browsers.</P>
<P>Note that this feature is exceedingly dangerous, as it will execute whatever 
code happens to be embedded in the <CODE>exec</CODE> tag. If you have any 
situation where users can edit content on your web pages, such as with a 
``guestbook'', for example, make sure that you have this feature disabled. You 
can allow SSI, but not the <CODE>exec</CODE> feature, with the 
<CODE>IncludesNOEXEC</CODE> argument to the <CODE>Options</CODE> directive.</P>
<HR>

<H2><A id=advancedssitechniques name=advancedssitechniques>Advanced SSI 
techniques</A></H2>
<P>In addition to spitting out content, Apache SSI gives you the option of 
setting variables, and using those variables in comparisons and 
conditionals.</P>
<H3><A id=caveat name=caveat>Caveat</A></H3>
<P>Most of the features discussed in this article are only available to you if 
you are running Apache 1.2 or later. Of course, if you are not running Apache 
1.2 or later, you need to upgrade immediately, if not sooner. Go on. Do it now. 
We'll wait.</P>
<HR>

<H2><A id=settingvariables name=settingvariables>Setting variables</A></H2>
<P>Using the <CODE>set</CODE> directive, you can set variables for later use. 
We'll need this later in the discussion, so we'll talk about it here. The syntax 
of this is as follows:</P><PRE>        &lt;!--#set var="name" value="Rich" --&gt;
</PRE>
<P>In addition to merely setting values literally like that, you can use any 
other variable, including, for example, environment variables, or some of the 
variables we discussed in the last article (like <CODE>LAST_MODIFIED</CODE>, for 
example) to give values to your variables. You will specify that something is a 
variable, rather than a literal string, by using the dollar sign ($) before the 
name of the variable.</P><PRE>        &lt;!--#set var="modified" value="$LAST_MODIFIED" --&gt;
</PRE>
<P>To put a literal dollar sign into the value of your variable, you need to 
escape the dollar sign with a backslash.</P><PRE>        &lt;!--#set var="cost" value="\$100" --&gt;
</PRE>
<P>Finally, if you want to put a variable in the midst of a longer string, and 
there's a chance that the name of the variable will run up against some other 
characters, and thus be confused with those characters, you can place the name 
of the variable in braces, to remove this confusion. (It's hard to come up with 
a really good example of this, but hopefully you'll get the point.)</P><PRE>        &lt;!--#set var="date" value="${DATE_LOCAL}_${DATE_GMT}" --&gt;
</PRE>
<HR>

<H2><A id=conditionalexpressions name=conditionalexpressions>Conditional 
expressions</A></H2>
<P>Now that we have variables, and are able to set and compare their values, we 
can use them to express conditionals. This lets SSI be a tiny programming 
language of sorts. <CODE>mod_include</CODE> provides an <CODE>if</CODE>, 
<CODE>elif</CODE>, <CODE>else</CODE>, <CODE>endif</CODE> structure for building 
conditional statements. This allows you to effectively generate multiple logical 
pages out of one actual page.</P>
<P>The structure of this conditional construct is:</P><PRE>        &lt;!--#if expr="test_condition" --&gt;
    &lt;!--#elif expr="test_condition" --&gt;
    &lt;!--#else --&gt;
    &lt;!--#endif --&gt;
</PRE>
<P>A <EM>test_condition</EM> can be any sort of logical comparison - either 
comparing values to one another, or testing the ``truth'' of a particular value. 
(A given string is true if it is nonempty.) For a full list of the comparison 
operators available to you, see the <CODE>mod_include</CODE> documentation. Here 
are some examples of how one might use this construct.</P>
<P>In your configuration file, you could put the following line:</P><PRE>        BrowserMatchNoCase macintosh Mac
        BrowserMatchNoCase MSIE InternetExplorer
</PRE>
<P>This will set environment variables ``Mac'' and ``InternetExplorer'' to true, 
if the client is running Internet Explorer on a Macintosh.</P>
<P>Then, in your SSI-enabled document, you might do the following:</P><PRE>        &lt;!--#if expr="${Mac} &amp;&amp; ${InternetExplorer}" --&gt;
        Apologetic text goes here
        &lt;!--#else --&gt;
        Cool JavaScript code goes here
        &lt;!--#endif --&gt;
</PRE>
<P>Not that I have anything against IE on Macs - I just struggled for a few 
hours last week trying to get some JavaScript working on IE on a Mac, when it 
was working everywhere else. The above was the interim workaround.</P>
<P>Any other variable (either ones that you define, or normal environment 
variables) can be used in conditional statements. With Apache's ability to set 
environment variables with the <CODE>SetEnvIf</CODE> directives, and other 
related directives, this functionality can let you do some pretty involved 
dynamic stuff without ever resorting to CGI.</P>
<HR>

<H2><A id=conclusion name=conclusion>Conclusion</A></H2>
<P>SSI is certainly not a replacement for CGI, or other technologies used for 
generating dynamic web pages. But it is a great way to add small amounts of 
dynamic content to pages, without doing a lot of extra work.</P></BODY></HTML>
